It sneaks up on you. You feel like your normal self and then, one day, you notice that your body shape has changed or that you’re holding on to a few extra pounds. Your body just doesn’t feel the same.

It’s not all in your head. As you get older, there are real changes in your body — some due to age, some due to menopause — that can lead to weight gain. But most women aren’t aware of them.

So, here’s what’s really going on with your body after 40 and what you can do to feel healthy and strong as you age.

The biggest culprit behind your body’s changes after 40? Hormones. These are the chemical messengers that control most body functions, from reproduction to hunger.

As you approach menopause, levels of estrogen, progesterone, and testosterone fluctuate, says Alyssa Dweck, MD, gynecologist and assistant clinical professor at the Mount Sinai School of Medicine.

This fluctuation in hormones causes a cascade of changes, from decreased bone density and lean muscle mass to lower sex drive and mood changes.

The fix: Don’t feel resigned to grinning and bearing the hormonal fluctuations! Talk to friends or family members, or find an online group. “You’ll probably hear that you’re all going through something similar.

When women hear positive stories and that this phase will end, it’s helpful,” says Amanda Thebe, a certified personal trainer and health coach who runs the Facebook group Menopausing So Hard.

Yes, you can blame your hormones for this, too. Not only does your resting metabolic rate naturally slow with age, lower estrogen levels contribute to a sluggish metabolism.

You also start to accumulate more fat, especially around your waistline, says registered dietitian Melissa Burton.

Researchers have found that hormonal changes caused by perimenopause and menopause contribute to changes in body composition, fat accumulation, and fat distribution.

The fix: The best way to keep your metabolism humming? Stay active.

Vera Trifunovich, a personal trainer and wellness coach at Uplift Studios, recommends a combination of strength training and cardiovascular exercise — something with a little impact, like cardio dance or a boxing class.

Plus, eat your fiber. While the average American eats 10 grams of fiber a day, you need between 25 and 35 grams, Burton says. Just make sure you drink plenty of water!

After the age of 40, you lose muscle mass — the main calorie-burning engine in your body — to the tune of 1 percent a year, Burton says. It’s linked to dropping estrogen and testosterone levels that accompanies perimenopause and menopause, says Dweck.

Coupled with a slower metabolism, you don’t burn calories the same way as you did when you were young.

Thebe’s workout plan

As you get older, and especially as you gain weight, the body starts to ignore insulin — the hormone responsible for regulating blood sugar levels.

As a result, you blood sugar is higher, because your cells aren’t absorbing it, says Burton. The result: It feels like you’re hungry, and you may experience more cravings.

Not only can this lead to unwanted pounds, it also puts you at greater risk for type 2 diabetes.

The fix: To avoid a glucose overload, Burton recommends including a mix of carbohydrates, protein, and fat at every meal.

Don’t just load up on carbs. “Protein and healthy fat help the body feel more satisfied for a longer period of time, and you don’t crave those super starchy carbs that can give you a sugar crash,” she says.

Pay attention to where your carbs come from, too. “If you drink juice, it increases blood sugar circulating in the body quickly,” Burton says. “If you eat whole grains, it has more fiber and breaks down slowly,” she says. It gradually releases sugar into the bloodstream.

Dweck suggests really sticking to a Mediterranean-style diet in your 40s. “It’s been shown to be protective against cancer and heart disease, and it doesn’t cause huge swings in blood glucose levels,” she says.

Hormones like ghrelin (which tells you when you’re hungry) and leptin (which tells you when you’re full) also fluctuate.

“As we age, the receptors for these hormones don’t work as well as they used to, and we become resistant to them, too,” Burton says. “It’s not just in your head. You’re actually hungry because of your hormones.”

The fix: Dweck suggests keeping a food diary to pinpoint pitfalls in your eating habits and to get a better handle on your hunger cues. “When you actually write down what you eat, you can see if you’re actually snacking all day or if you’re eating bigger portions,” she says.

A food diary can also tell you if you’re eating enough protein. Burton recommends 20 to 30 grams of protein at each meal, since your body can absorb only so much protein in one sitting.

Sometimes tackling changes isn’t about doubling down with the same routine, but finding a new one that works for you.

Christine Yu is a freelance writer, covering health and fitness. Her work has appeared in Outside, The Washington Post, and Family Circle, among others. You can find her on Twitter, Instagram, or at christinemyu.com.